Are you saying the content of his preaching is equivalent in its falseness to Mormons? It's been years since I've listened to one of his sermons.
Joel Osteen got his basic theology from his father, John Osteen. John Osteen was
a health and wealth "seed faith" preacher. The health and wealth teaching is directly tied to the New Thought movement out of Boston, Massachusetts, from the teachings of
Phineas P. Quimby in the early 19th century. He taught that reality was created by the mind, and positive confession creates one's reality. It became quite popular, and
Christian Science, Unity, and a number of other explicitly non-Christian movements came out of it, all pushing health and wealth positive thinking.
E.W. Kenyon, a Baptist minister in Massachusetts, took Quimby's teachings and fitting "Christian" terminology around it. Fortunately, for a time, Kenyon's writings were somewhat obscure, but
Kenneth Hagin found them and boldly plagiarized Kenyon's writings, presenting them as revelation from God, and introduced them to the charismatic movement.
Those teachings became extremely popular because the theology was inspired by a con man (Quimby) to separate people from their money, so many charismatic preachers and con men began teaching it get rich.
Getting back to Joel Osteen... he absorbed his father's theology, then added the positive thinking aspect to it, creating his own blend of heresy.
Yes, I believe Osteen is not much better than the Mormons. In fact, I think he is a little more dangerous, since he is thought to be an explicitly Christian leader and he is distributing poison. Mormons are quirkier with their additional "revelation" and three extra books of scripture that they have to 'sell' people before they can make a convert.
Are there any contemporary preachers who focus on the power of positive thinking who don't have a questionable theology?
I don't know of any.